U.S. Pat. No. 2,563,502 of Benjamin J. Sweo discloses a process for preparing a fired coating of porcelain enamel which utilizes a ceramic slip containing non-metallic inorganic fibrous material. The fibers used in the Sweo process have a length of from about 20 to about 100 microns (see lines 28-31 of column 2).
The process of the Sweo patent, however, does not produce a fired coating with sufficient impact resistance. Thus, in his U.S. Pat. No. 4,407,868 (and in the documents contained in the file history of such patent), Yoshirhiro Iizawa discloses that it is essential to use substantially longer fibers than those used in the Sweo process. Thus, e.g., in a "DECLARATION UNDER RULE 132" signed on Dec. 21, 1982, Iizawa discussed a test in which two types of glass lined specimens (Specimens A and B) were prepared. Specimen A contained fibers having a diameter of 10 microns and a length of 100 microns in an amount of 5% by weight (in accordance with the teaching of Sweo U.S. Pat. No. 2,563,502). Specimen B contained fibers having a diameter of 100 microns and a length of 1,000 microns in an amount of 5 weight 1 percent. Both specimens were tested for impact resistance.
On the last page of his "DECLARATION. . . ," Iizawa stated that ". . . the reinforcing effect by fibers of Specimen A containing fibers having a length of 100 microns (Sweo's Patent) was about one tenth of that of Specimen B containing fibers having a length of 1,000 microns . . . . "
As a result of this disclosure, Iizawa's patent application was allowed with claims calling for the use of a slip comprised of inorganic fibers with a length of from 1 to 5 millimeters.
Although the coatings produced by the Iizawa process reportedly have better impact resistance than those produced by the Sweo process, they have poor chemical resistance. Thus, e.g., the coatings of the Iizawa patent, when exposed to hydrochloric acid in a 20 volume percent vapor phase, lose from about 0.9 to about 1.0 grams of weight per square meter per day (see TABLE 7). This weight loss due to exposure to acid is unacceptably high for chemical processing equipment.
It is an object of this invention to provide a process for producing a chemically resistant porcelain enamel coating with good impact resistance and acid resistance which is substantially superior to that possessed by the coatings of the Iizawa patent.
It is another object of this invention to provide a chemically-resistant porcelain enamel in which crack propagation is minimized.
It is yet another object of this invention to provide a process for preparing a chemically-resistant porcelain enamel coating which is substantially smoother than many prior art enamel coatings. This property will facilitate non-adhesive contact between the enamel coating and polymeric/elastomeric bodies which often occurs during chemical reactions.